So I called a buddy of mine who runs a battery shop to see what he had available. I run Deka batteries, he does not sell Deka. He said since I only need one battery, I should absolutely stick with Deka. Apparently each battery has internal resistance particular to its manufacturer, and mixing brands on a series circuit of 36 volts can create problems. He said I will either damage the new one, or damage the remaining two. Voltage takes the path of least resistance, so whichever battery has the lower resistance will end up getting overcharged while the remaining batteries are charging. So I guess I will get another Deka.
Moral of the story: Do not mix battery brands in a series circuit.